The present invention relates to power amplifiers. In particular, the invention relates to a system and apparatus utilizing a multiplexed input modulation scheme for linear high efficiency power amplification.
The present invention also relates to efficient amplification of signals whose envelope is amplitude modulated, and specifically the application of linear amplification at RF frequencies of such signals up to high power levels. In applications that are constrained by limited available DC power (say from a fixed capacity battery power supply), there is a challenge to tailor the waveforms present at the active device output (at any given stage along the chain of amplification) such that power is not dissipated within the active device and is retained for the conversion from DC to RF signal energy. The less power dissipated within the active device, the higher the efficiency of the amplifier. Dissipated power within the active device occurs when there is a simulataneous overlap of non-zero voltage and current of the carrier signal at the output terminal of that device. This product of non-zero voltage and current is wasted energy that detracts from the intended output signal power at the carrier frequency and degrades efficiency. Various techniques and amplifier topologies exist which minimize the finite overlap of current and voltage to a varying degree, and these may be applied in combination with the proposed invention for maximum efficiency when finite amplitude modulation is required.
In applications that also require linear amplification of desired signals, the same clipping that enhances efficiency may degrade linearity. The requirement for an amplified replica of the input signal is difficult to achieve if the waveform is "clipped" or distorted.
This distortion affects certain aspects of the input signal more than others. Just as the input signal can be described to have an amplitude modulation and phase modulation, the distortion can be described as a matrix which when multiplied by those amplitude modulation and phase modulation components will yield the resultant amplitude modulation and phase modulation components at the output. For example, ##EQU1##
For certain signals which do not contain any amplitude modulation and only carry information through PM information, the only term of significance in this equation is the PM.sub.o /PM.sub.i term. Often the phase changes induced from a purely phase-modulated input signal do not induce large phase distortion even in very nonlinear amplifiers. Accordingly, a very efficient nonlinear power amplifier can be used to amplify the phase-modulated input without significant degradation in the critical phase information.
This is not the case for amplitude-modulated signals, however, wherein a large envelope amplitude can induce significant amplitude distortion as a result of amplifier nonlinearities. Large changes in amplitude at the device input typically cause large changes in the device capacitances and conductances, all of which vary nonlinearly. These nonlinear conductance and capacitance changes result in significant distortion of the waveform seen at the output. This distortion ultimately degrades the ability of the power amplifier to meet the linearity requirements of its application. At the same time, the efficiency benefits of operating an amplifier nonlinearly are significant, and thus the use of nonlinear amplifiers to linearly amplify signals with strong amplitude modulation has been an area of much study.
Established approaches exist to restore the amplitude modulation envelope on the output of a nonlinear amplification stage, which is itself able to achieve a very high efficiency, and therefore the system solution modulating that core amplifier is able to attain high efficiency. Such approaches generally rely on the use of a phase-modulated signal of constant envelope at the amplifier input. The nonlinear amplifier can efficiently amplify the phase modulation component without AM/AM and AM/PM distortion. The amplitude modulation envelope information is then restored at the amplifier output without inducing amplitude distortion at that stage.
One well-known approach for separately amplifying amplitude and phase information is Linear Amplification Using Nonlinear Components (LINC). The technique utilizes a pair of amplifier chains, each operating on constant-envelope signals whose relative phase is varied such that their sum results in a desired envelope having varying amplitude. The power combining of the two chains at their output sums the coherent parts of each waveform and places that result on the output. The destructive interference between the two signals is dissipated in the termination resistor of the power combining element. This dissipation can significantly degrade the efficiency of the overall amplifier, especially when the two signals are significantly out of phase in order to reach a minimum desired output amplitude.
Envelope Elimination and Restoration (EER), or Kahn-technique transmission, is also well-known. Like LINC, EER involves the use of a constant envelope phase modulated input signal. EER, however, restores the amplitude modulated envelope information directly on the supply line of the output DC supply. By directly modulating the supply voltage on the output, the resultant waveform consists of the amplified phase information to a saturated level defined by the supply voltage. The output then restores the envelope by becoming the upper and lower limits within which the amplified phase waveform is bracketed. Modulation of the supply voltage can be problematic, however, as the spurious output of the switching regulator can interfere significantly with the desired envelope. Furthermore, to minimize the effect of the switching regulator interference, it must be switched at a high rate and the intermodulation spurs at its output must be filtered down to very low frequency. As a result, if a feedback loop is required in order to correct for amplitude distortion, the bandwidth of that loop is constrained to be extremely small and unusable for wide bandwidth channel communications.
There is therefore a need for an improved envelope restoration scheme that overcomes the limitations of power combining losses in LINC, and the switching regulator limitations in standard EER/Kahn implementations.